A service of a digital television broadcast is promoted lately and it is expected to receive it by a portable telephone or a portable terminal device. And in the field of portable terminal devices, a diversification of devices has been attempted by providing a function for receiving frequency bands of various fields such as not only those of a cellular communication, but also those of a GPS (a global positioning system; 1.5 GHz band), and a Bluetooth (2.45 GHz band). Therefore, an antenna for such a portable wireless device is also expected to have a simple structure and operate in more multiple and wider frequency bands.
As conventional variable tuning antennas of a simple structure operating in a wide band, an antenna shown in FIG. 6A is known (for example, cf. Japanese Patent Application Laid-Open No. 2002-232313 (FIG. 1)). In this antenna, a variable capacitance element 52 is connected between a base terminal part of a radiation element 51 such as a monopole type or a helical type and a feeding part, and by utilizing a series resonance of a capacitance of the variable capacitance element 52 and an inductance of a radiation element 51, a tuning frequency is varied by varying the capacitance of the variable capacitance element 52 with voltage applied to it. Further, an antenna shown in FIG. 6B is known (for example, cf. Japanese Patent Application Laid-Open No. HEI10-209897 (FIG. 1)). In this antenna, a fixed inductance element 53 and a variable capacitance element 52 are connected to a radiation element 51 in series and the series resonance frequency is varied by varying the capacitance of the variable capacitance element 52.
Resonance frequencies (angular frequency ω) in the structures shown in FIG. 6A and in FIG. 6B described above are represented as following equations (1) and (2) respectively. In other words, as the radiation element 51 is inductive and its capacitance is very small, taking La for the inductance of the radiation element 51, C for the capacitance of the variable capacitance element 52 and L for the inductance of the fixed inductance element 53, two equations are represented as follows.ω=1/(La·C)1/2  (1)ω=1/{(La+L)·C)}1/2  (2)
In equation (1), as La, an inductance of a radiation element, is nearly constant in most structures of radiation elements,. ω (a resonance angular frequency) varies linearly according to C1/2 with a variation of a capacitance of a variable capacitance element. But, in general, because variable capacitance elements currently available have a limitation in a range of a variation of their capacitance such that the maximum amount of the variation is only up to about 150 MHz, for example, in a low frequency like that of a digital TV band (470-770 MHz), problems may arise that tuning in whole frequency band (fL-fH) of digital TV cannot be achieved and that a characteristics of VSWR and an antenna gain deteriorate in the certain frequency bands, as explained in FIG. 7 where an example of the relationship between the VSWR and the gain to a frequency is shown in case of varying a capacitance of a variable capacitance element.
On the other hand, in a case of a structure shown in FIG. 6B, it is obvious from equation (2) that the resonance frequency ω is (La+L)−1/2/La−1/2 times, that is {La/(La+L)}1/2 times to that given by equation (1), assuming that the C of a variable capacitance element is the same in these equations, and that as L is positive, the coefficient described above is always smaller than 1, and that the amount of the variation of the resonance frequency is smaller than that shown in equation (1) with the same amount of the variation of the capacitance C. Although, there are cases that the amount of the variation of a resonance frequency in the structure shown in FIG. 6B becomes greater due to the stray capacitance formed between an antenna element or a tuning circuit and the ground in a practical case, but it does not make big difference.